Watching a friend or relative in a stressful situation can loosely synchronize both of your heart rates, experiments at a fire walking ritual suggest.

In the experiment, when a spectator observed a relative or friend walk across hot coals, both the onlooker and performer's heart rates changed at the same time, though they didn't match each other beat for beat.

Past studies have observed that sports fans' hearts race when their teams score, but no one had yet delved into the physical effects on both spectators and participants, according to study co-author Ivana Konvalinka, a Ph.D. student at the Center of Functionally Integrative Neuroscience at Denmark's Aarhus University.

The finding suggests social bonds between people may manifest themselves even more powerfully than thought, Konvalinka noted.

The results show that "we can find markers of emotional connectedness in bodily measures as well—it's not just a cognitive effect," Konvalinka said.

Konvalinka suspects synchronization may occur between friends or relatives during other stressful or emotional events, such as weddings.

The team put heart rate monitors on 12 fire walkers, 9 spectators who were relatives or friends of at least one fire walker, and 17 spectators with no connection to the fire walkers.

Advanced statistical analyses revealed that the heart rates of relatives and friends followed similar patterns as those of the performers. No such effect existed in onlookers who did not know the performers.

It's unknown how this mechanism actually occurs, noted Konvalinka, whose study appeared this week in the journal Proceedings of the National Academy of Sciences. For instance, part of the phenomenon may be because related people have similar heart rates. However, this is not enough to explain the entire effect, she said—for example, why similar effects were seen in friends.

The discovery that people's hearts can harmonize solely on visual or auditory information reinforces a law of nature, according to Michael Richardson, a psychologist at the University of Cincinnati in Ohio.

The natural law of coupled oscillators holds that when two or more rhythms meet, they will become coordinated—a phenomenon seen across the natural world, from fireflies matching their flashes to groups falling into step.